New Warlock Sub Class Suggestion - Servant of Sin

Wow, I love the background story! Stays true to the original character. The original character was half rogue/half warlock, so this keeps him more in one basket. I also like how Greed shows up, because in our campaign, Greed also showed up to pick up Daxis :P My only quibble is twofold: that Daxis is lawful/evil and the real character was pact of the tome. Daxis was sort of the linguist of the party, able to communicate and use his magical powers. As for alignment, I say lawful evil because the original character had a strict sense of loyalty, and that he would defend anyone that defended him, unless lord Greed said otherwise. This is born from his father figure, Hux. Also, he hates all Dark Folk and enjoys killing them. But, you can do whatever you want really because this is amazing :P Will there be any artwork?

Certainly I can make those two changes. Iím glad you like it. I tried to keep it as close to your background as possible while still fitting it into Farland canon. I took the appearance of Greed from your tale. And there will definitely be art.

Pride- Lash Out: I would be tempted to just make this into "Starting at 6th level, when you are damaged by an attack, as a reaction you may cast hellish rebuke without using a spell slot. You may not Lash Out again until you have completed a short or long rest

That way it scales with level a little better and does not require a weapon out etc. The spell also provides a range, damage, etc so it does not need to be listed in the skill.

Gluttony-Insatiable Hunger I would just change the wording to "choose a living creature you can see that needs to eat."

Envy- Imitate Magic Just a note that I have not really checked this for balance but as we ran into with the Aid spell, multiclass long duration spells combined with Warlock ability to regain slots on rest can do weird things. There is nothing crazy I know right now but it might be worth a reminder of DM fiat.

Sloth- Quagmire Stun 5 creatures for a round an a half at level 6? I know it is a once a day but it still seems strong, I feel like at least if it is only until the start of next turn it might be better. Perhaps a way to scale based on level might be better.

At 6th level, you can radiate lethargy. As an action, target 5 enemies that you can see within 30 feet. Those enemies must make a Wisdom saving throw against your Warlock spell save DC, gaining two levels of exhaustion on a failure or one level on a success. When you use your quagmire ability, all terrain within 30 feet of you is treated as difficult terrain for everyone except you until the end of your next turn. You cannot use your quagmire ability again until you have completed a long rest.

I like using exhaustion but I am not sure a 24 hour halving of speed is weaker than stun. How about leaving it as is but say it stuns a number of creatures equal to your proficiency modifier. So at level 6 it stuns 3 and at level 20 it stuns 6. That way you you do not immobilize an entire party at level 6 but it scales a bit.